Greetings, and thanks for your reply.

No, I’m not using any Nemedian foundations (although I have them available, they aren’t part of my base design).
What I’m using is:

  1. Brick pieces (when you are at the Place of Summoning, going left and up that ledge, there’s a cliff. It’s extending almost all the way around it.
  2. Hardened brick pieces (the aforementioned ledge to the left of the Place of Summoning)
  3. On the inselberg to the Northwest of the Point of Summoning (the one right south of the Hand of the Creator) it’s Aquilonian pieces (this is connected to the part at the Place of Summoning via a bridge constructed from hardened brick).
  4. A bridge constructed from hardened brick leading to the mountain next to Thugra’s Stand.

However, as it seems things get corrupted when exceeding a certain amount of elements (I’d have to estimate how much I have placed already, but if you are placing too many, something gets corrupted, causing the already-placed base components - foundations, walls, etc. - to vanish).
I’m continuing my investigations right now (i. e. place a few additional elements, then quit the game to get a save, restart and see what’s happening). If my guess is right, I should be hitting the same problem very soon now.
Once I know more, I’m going to add screenshots.

EDIT: My suspicions turned out to be right. I have just placed six additional pieces, and things got corrupted.